The OLED is a flat panel display technology which has great prospects for development. It possesses extremely excellent display performance, and particularly the properties of self-illumination, simple structure, ultra thin, fast response speed, wide view angle, low power consumption and capability of realizing flexible display, and therefore is considered as the “dream display”. Meanwhile, the investment for the production equipments is far smaller than the TFT-LCD. It has been favored by respective big display makers and has become the main selection of the third generation display element of the display technology field. At present, the OLED has reached the point before mass production. With the further research and development, the new technologies constantly appear, and someday, there will be a breakthrough for the development of the OLED display elements.
The OLED can be categorized into two major types according to the driving ways, which are the Passive Matrix OLED (PMOLED) and the Active Matrix OLED (AMOLED), i.e. two types of the direct addressing and the Thin Film Transistor matrix addressing. The AMOLED comprises pixels arranged in array and belongs to active display type, which has high lighting efficiency and is generally utilized for the large scale display devices of high resolution.
At present, the AMOLED gradually becomes mature. In the AMOLED, it requires the current for driving. The Low Temperature Poly-Silicon (LTPS) has larger mobility, and the Thin Film Transistor (TFT) manufactured with the active layer can satisfy the current drive mode of the AMOLED. The Low Temperature Poly-Silicon Thin Film Transistor (LTPS TFT) has higher mobility and can achieve the higher on state current. However, the defect due to the grain existence in the LTPS will lead to the appearance of the higher off state current as the LTPS TFT is in off state. For decreasing the off state current of the LTPS TFT, the Lightly Doped Offset structure can be utilized. There are more researches about the Offset structure at present. Nevertheless, the offset structure forms the high resistance region which will reduce the on state current of the LTPS TFT. For gaining the higher on state current, the improvement can be performed to the offset structure.
In the LTPS TFT having the offset structure, for reducing the influence of the high resistance region to the on state current, the overlap structure can be added in the TFT. Namely, the source or drain has an overlap region with the gate, which can decrease the channel length and reduce the high resistance region, and to promote the on state current. In the LTPS TFT, the source or drain respectively utilize the offset structure and the overlap structure, which can realize the smaller off state current, and meanwhile the influence to the on state current is less. However, in one LTPS TFT, as the source and drain respectively utilize the offset structure and the overlap structure, it leads to the asymmetry of the TFT structure. In the practical work of the TFT, as the source and the drain are exchanged, the current flowing through the element is influenced due to the asymmetry of the element.